A variable length connecting rod which can change the effective length comprises a connecting rod body, an eccentric member which is attached at the small end to the connecting rod body to be able to swivel and which changes the effective length of a variable length connecting rod when swiveled, one piston mechanism which has one hydraulic cylinder which is provided in the connecting rod body and one hydraulic piston which slides in the hydraulic cylinder, a direction switching mechanism which switches between supply and discharge of hydraulic fluid to the hydraulic cylinder, and a hydraulic fluid supply source which supplies the hydraulic cylinder with hydraulic fluid. The piston mechanism is configured so that the eccentric member swivels in one direction if hydraulic fluid is supplied to the hydraulic cylinder and so that the eccentric member swivels in the other direction if hydraulic fluid is discharged from the hydraulic cylinder.